In battery systems utilized for applications that require high power and high capacity batteries such as hybrid electric vehicles (HEV) and electric vehicles (EV), a plurality of battery cells are connected in series or in parallel for use. In such battery systems, voltages and currents of the battery cells are monitored in a battery ECU (Electronic Control Unit) in order to protect each battery cell by monitoring abnormalities of each battery cell such as excess current, overdischarge, overcharge, variation in an SOC (an index representing a charge state on percentage) of the battery cell. There has been proposed a battery system (Patent Document 1) as such a battery system. In the proposed battery system, a battery module is configured to include a voltage detecting circuit for detecting a voltage of a battery block having a plurality of battery cells connected in series, and the voltage detecting circuit provided for each battery module and the battery ECU provided for a plurality of battery modules are connected to each other by a communication line, so that the voltage of each battery block can be monitored.
[Patent Document 1] JP 8-162171 A